The present invention relates generally to magnetic resonance imaging, and, more particularly, to a system and method for parallel imaging with segmented, accelerated acquisition. Embodiments of the system and method may utilize segmented, rotated, and/or non-Cartesian k-space trajectories such as the Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) technique to acquire multi-channel blades of k-space data. Oversampled “calibration” regions of the blades, or of a reference blade, may then be used to determine the parallel imaging reconstruction weights, used for synthesis of additional k-space data points.
MR imaging in general is based upon the principle of nuclear magnetic resonance. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
One technique for quickly acquiring a set of MR signals is parallel imaging. Broadly, parallel imaging utilizes an array of RF coils to each acquire a view of the scan subject. By simultaneously acquiring multiple channels of data, it is possible to increase the step size between phase-encoding lines, or equivalently to reduce the size of the field of view and the amount of data collected. In other words, scan time reduction is achieved by under-sampling k-space and recording k-space data simultaneously from the multiple imaging or receive coils. Under-sampling generally reduces the data acquisition time by increasing the distance of sampling positions in k-space.
Parallel imaging techniques not only expedite data acquisition, but also reduce aliasing or wrapping that occurs in the phase-encoding direction when an imaging object extends outside the reduced field-of-view (FOV). In particular, parallel imaging techniques remove or reduce the aliasing by using coil sensitivity maps or calibration data, to define or determine an unaliased spin distribution. Information regarding the coil B1 sensitivities is typically acquired with an external calibration or a self-calibration technique. One method for parallel imaging using externally-generated sensitivity maps is known as SENSitivity Encoding (SENSE). An exemplary method for parallel imaging using self-calibration is known as GeneRalized Autocalibrating Partially Parallel Acquisition (GRAPPA). Generally, the coil sensitivity or calibration data is used to reduce aliasing in the reconstructed image that can occur as a result of under-sampling.
However, when certain imaging sequences are performed via a parallel imaging system, it can be difficult to achieve these undersampling or “acceleration” benefits. As an example, for the PROPELLER technique, in which k-space blades are acquired at various angles rotated about the k-space center, high net acceleration factors are presently not known to be implementable. While parallel imaging can shorten the echo train length, increase blade width, or reduce the number of acquired blades, these advantages are counterbalanced by several factors. In techniques which use externally-determined sensitivity data, it can be difficult and/or time-consuming to obtaining accurate sensitivity maps, especially where subject motion is an issue. In some instances, autocalibrating techniques can thus be more advantageous when motion insensitivity is a goal.
For autocalibrating techniques, however, the increase in scan time when separate calibration data is acquired for each blade can significantly reduce the net acceleration provided by parallel imaging. It would therefore be desirable to have a system and method capable of maintaining the acceleration benefits of parallel imaging while utilizing the motion insensitivity of segmented, rotated acquisition techniques like PROPELLER.